1. Field of the Disclosure
The present disclosure relates to a method of forming a channel region of a thin film transistor composed of monocrystalline silicon, and more particularly, to a method of forming a high quality channel region of a thin film transistor (TFT) by forming a large monocrystalline silicon thin film using a patterned metal mask and a grain boundary filter region.
2. Description of the Related Art
Of the elements that form a flat display device, such as a TFT LCD, the core element is the TFT which is a switching device. Generally, a channel of a TFT is formed of amorphous silicon or crystalline silicon obtained by crystallizing amorphous silicon. That is, as depicted in FIG. 1A, after forming a buffer layer 12 formed of silicon oxide on a substrate 11 formed of glass, silicon, or plastic, an amorphous silicon layer 13 is formed on the buffer layer 12. Next, a TFT 14 as depicted in FIG. 1B is formed by patterning and doping the amorphous silicon layer 13. In this case, a channel region of the TFT is amorphous silicon.
However, the high-speed operation of a device is difficult since amorphous silicon (a-Si) has low charge mobility. Accordingly, the amorphous silicon cannot be applied to a high-resolution display panel. To solve this problem, the TFT 14 depicted in FIG. 1B can be formed after forming polycrystalline silicon by crystallizing the amorphous silicon layer 13 depicted in FIG. 1A. In this case, a channel region of the TFT 14 is polycrystalline silicon. Polycrystalline silicon (poly-Si) has a charge mobility of more than 100 times faster than that of amorphous silicon (a-Si). Due to this advantage, a driving circuit can be built directly on a display panel. Therefore, manufacturing costs can be reduced and a slim and high-resolution large screen display can be produced.
Methods of crystallizing an amorphous silicon thin film deposited on a substrate are an excimer laser annealing (ELA) method and a solid phase crystallization (SPC) method. Recently, improved ELA methods, such as a metal induced lateral crystallization (MILC) method or a continuous grain solidification (CGS) method are also used. These are methods for crystallizing an amorphous silicon thin film to a polycrystalline silicon thin film.
However, even in the polycrystalline silicon thin film, the flow of charges is interrupted by many grain boundaries. Accordingly, to obtain superior electrical characteristics, the entire region on which a channel of a TFT is formed must be formed of monocrystalline silicon.
FIGS. 2A through 2C illustrate conventional methods of crystallizing silicon to form monocrystalline silicon, and FIG. 2D is a drawing illustrating a mask used in the crystallization methods of FIGS. 2A through 2C.
Referring to FIG. 2D, in a mask 15, a plurality of first and second apertures 16 and 17 having a rectangular shape spaced at a predetermined distance in a vertical direction are formed in two columns. As illustrated, the first and second apertures 16 and 17 are disposed in a zigzag shape.
After disposing this type of mask 15 a predetermined distance above amorphous silicon, the amorphous silicon under a region corresponding to the first and second apertures 16 and 17 melts when a laser beam is vertically irradiated through the mask 15. When the amorphous silicon irradiated by the laser beam is completely melted, the irradiation of the laser beam is terminated. Then, as depicted in FIG. 2A, crystallization begins from an outer boundary surface and proceeds toward an inner side of the melted amorphous silicon while the melted amorphous silicon is being cooled. At this time, a grain boundary of the silicon crystal proceeds in a direction of approximately 90° with respect to the outer boundary surface of the melted silicon. FIG. 3A is a photograph of a crystallized region indicated by “A” in FIG. 2A. Referring to FIG. 3A, the crystallization has proceeded from both an upper boundary surface and a lower boundary surface of melted silicon toward a central portion thereof. As a result, fine boundaries are vertically formed, and a bold grain boundary in a horizontal direction is formed at the central portion where crystallization from two directions meets at the central portion.
Next, as depicted in FIG. 2B, the mask 15 is horizontally moved, and the amorphous silicon between the regions on which the crystals have been formed is melted. That is, the amorphous silicon between the regions melted by laser beams passed through the second aperture 17 of the mask 15 in FIG. 2A is melted by laser beams passed through the first aperture 16 of the mask 15 in FIG. 2B. At this time, outer boundaries of the melted regions contact with or are partly overlapped by regions already crystallized in FIG. 2A. Then, the amorphous silicon is crystallized as depicted in FIG. 2C when the melts are slowly cooled by terminating the irradiation of the laser beams. That is, the crystallization proceeds continuously from crystals of the already crystallized region. FIG. 3B is a photograph of a crystallized region indicated by “B” in FIG. 2C. As depicted in FIG. 3B, large and continuous grains, in which grain boundaries are formed in one direction, are grown.
FIG. 4 is a cross-sectional view illustrating another conventional method of crystallizing silicon using an aluminum (Al) mask. Referring to FIG. 4, an amorphous silicon layer 21 is formed on an oxide substrate 20 such as silicon oxide, and a mask 23 formed of aluminum (Al) is partly formed on the amorphous silicon layer 21. Afterward, laser beams are vertically applied onto the amorphous silicon layer 21 using an excimer laser. Accordingly, a portion of the amorphous silicon layer 21 covered by the aluminum mask 23 is not melted and the remaining portion of the amorphous silicon layer 21 exposed to the laser beams is melted. When the irradiation of the laser beams is ended, crystallization proceeds from boundaries between the aluminum mask 23 and the amorphous silicon layer 21 inward to the melted amorphous silicon layer while heat is discharged rapidly through the aluminum mask 23, which has a relatively high thermal conductivity.
According to the method of crystallizing amorphous silicon described above, relatively large-sized of monocrystalline silicon can be obtained. However, this method cannot completely remove grain boundaries and many minute grain boundaries remain in the crystal. More specifically, the charge mobility in a vertical direction to the grain boundary is very low.